1. Field of the Invention
The present invention relates to an output control method for pulse arc welding, whereby a welding power source provides the desired external characteristics having a required slope Ks.
2. Description of the Related Art
In the case of pulse arc welding using a consumable electrode, it is extremely important to maintain the arc length during welding at an appropriate value in order to improve the welding quality, e.g., beautiful external appearance of the bead and uniform welding penetration depth. Generally the arc length is determined by the balance between the wire feeding rate and the fusion rate. Accordingly, if the fusion rate, which is substantially proportional to the mean value of the welding current, is equal to the feeding rate, the arc length always remains constant. However, the feeding rate during welding fluctuates as a result of e.g. fluctuations in the rotational speed of the feeding motor, and fluctuations in the frictional force along the feeding path as the welding torch cable is pulled around. As a result, the balance with the fusion rate breaks down so that the arc length varies. Furthermore, the arc length also fluctuates as a result of e.g. fluctuations in the distance between the torch and the matrix due to movement of the hands of the welding operator, or irregular vibration of the welding pool. In order to suppress fluctuations in the arc length caused by such various causes of fluctuation (hereafter referred to as “disturbances”), it is necessary to perform arc length control by constantly adjusting the fusion rate in accordance with the disturbance so that variation of the arc length is suppressed.
In the case of consumable-electrode gas shielded arc welding, among which is consumable-electrode pulse arc welding, a method in which the external characteristics of the welding power source is controlled to desired values is commonly used for suppressing fluctuations in the arc length caused by the disturbances described above. FIG. 9 shows an example of external characteristics. The horizontal axis in the figure shows the mean value Iw of the welding current flowing through the welding wire, while the vertical axis shows the mean value Vw of the welding voltage applied between the welding wire and the matrix. The characteristic L1 shows constant-voltage characteristics in which the slope Ks=0 V/A. The characteristic L2 shows voltage characteristics with the slope Ks=−0.1 V/A, descending to the right. The external characteristics can be expressed as a straight line. Thus, external characteristics passing through the intersection point P0 of the reference value Is of the welding current and the reference value Vs of the welding voltage, and having a slope of Ks, can be expressed by the following equation:Vw=Ks×(Iw−Is)+Vs  Equation (1)
It has been widely known that the stability of arc length control (called the self-controlling effect) is greatly affected by the slope Ks of the external characteristics of the welding power source. In order to stabilize the arc length against disturbances, it is necessary to control the slope Ks of the external characteristics to an appropriate value in accordance with the welding conditions, including the welding method. For example, in the case of a carbon dioxide gas arc welding method, an appropriate value of the slope Ks is in the range of approximately 0 to −0.03 V/A, while in the case of a pulse arc welding method, such a value is in the range of approximately −0.05 to −0.3 V/A. Accordingly, in the pulse arc welding method that is the object of the present invention, it is necessary to provide the characteristic L2 or the like, with a predetermined slope Ks in the range of approximately −0.05 to −0.3 V/A instead of the characteristic L1, in order to stabilize the arc length control. A conventional method for attaining the external characteristics having the desired slope Ks in pulse arc welding will be described below.
FIG. 10 shows current and voltage waveform diagrams for pulse arc welding. FIG. 10(A) is a waveform diagram of the welding current (instantaneous value) io, and FIG. 10(B) is a waveform diagram of the welding voltage (instantaneous value) vo. The following description will make reference to the figure.
(1) Peak Period Tp from Time t1 to t2
During the predetermined peak period Tp, as shown in FIG. 10(A), a predetermined peak current Ip with a large current value is caused to flow in order to cause a transition of the welding wire into molten droplets. Furthermore, as shown in FIG. 10(B), a peak voltage Vp is applied that is substantially proportional to the arc length during this period.
(2) Base Period Tb from Time t2 to t3
During the base period Tb that is determined by the welding power source output control that will be described later, as shown in FIG. 10(A), a predetermined base current Ib with a small current value is caused to flow in order to prevent the growth of a molten droplet at the tip end of the welding wire; furthermore, as shown in FIG. 10(B), a base voltage Vb is applied that is substantially proportional to the arc length during this period.
Welding is performed by repeating a period from time t1 to t3, comprising the peak period Tp and the base period Tb, as one pulse period Tpb. As shown in FIG. 10(A) the mean value of the welding current for each pulse period Tpb is Iw. Similarly, as shown in FIG. 10(B), the mean value of the welding voltage for each pulse period Tpb is Vw. The output control that is performed in order to form the external characteristics of the welding power source is accomplished by performing feedback control with the time length of the pulse period Tpb used as an operating quantity. Output control is performed by setting the peak period Tp as a fixed value, and increasing or decreasing the pulse period Tpb.
As shown in FIG. 11, the mean value of the welding current for the n-th pulse period Tpb(n) from time t(n) to t(n+1) is Iw(n), and the mean value of the welding voltage for this pulse period is Vw(n). In FIG. 9 as described above, output control is performed so that the intersection point P1 between these values Iw(n) and Vw(n) is located on the characteristic L2. Below, a welding power source output control method that is used to provide external characteristics with a desired slope Ks will be described.
(1) As shown above in FIG. 9, when the slope Ks, welding current reference value Is and welding voltage reference value Vs are set beforehand, the target external characteristics to be provided is expressed by the Equation (1). As shown above in FIG. 10, the peak current Ip, base current Ib and peak period Tp are set beforehand at fixed values.
(2) A first variable A=Ks×(Ib−Is) and a second variable B=Ks×(Ib−Ip)×Tp are calculated.
(3) The welding voltage vo during welding is detected.
(4) The integration Sva=∫(A+Vs−vo)dt of the slope forming voltage error is calculated from the starting time point of the n-th pulse period Tpb(n), and the n-th pulse period Tpb(n) is ended at the time point at which the integration Sva of the slope forming voltage error in the base period following a predetermined pulse period becomes equal to or greater than the value of the second variable B (Sva≧B).
(5) As described above, the external characteristics by Equation (1) can be provided by performing output control of the welding power source.
FIG. 12 is a block diagram of a conventional welding power source provided with an output control function for the purpose of attaining the external characteristics noted above. The blocks will be described below with reference to the figure.
The main power supply circuit, upon receiving a commercial alternating power (three-phase 200 V, for example), performs power control, such as inverter control, in accordance with a current error amplifying signal Ei to be described later, and outputs a welding current io and welding voltage vo that are suitable for welding. The welding wire 1 is supplied via the interior of a welding torch 4 by the rotation of the feeding roll 5 of a wire supply device, and an arc 3 is generated between this welding wire and a matrix material 2. The voltage detection circuit VD detects the welding voltage vo, and outputs a voltage detection signal vd.
The welding current reference value setting circuit IS outputs a predetermined welding current reference value setting signal Is. The welding voltage reference value setting circuit VS outputs a predetermined welding voltage reference value setting signal Vs. The peak current setting circuit IPS outputs a predetermined peak current setting signal Ips. The base current setting circuit IBS outputs a predetermined base current setting signal Ibs. The peak period setting circuit TPS outputs a predetermined peak period setting signal Tps. The first variable calculating circuit CA calculates Ks×(Ibs−Is), and outputs a first variable calculated value signal Ca. The second variable calculating circuit CB calculates Ks×(Ibs−Ips)×Tps, and outputs a second variable calculated value signal Cb. The slope forming voltage error integrating circuit SVA performs the integration of ∫(A+Vs−vo) dt from the starting time point of the n-th pulse period Tpb(n), and outputs a slope forming voltage error integration signal Sva. The comparison circuit CM outputs a comparative signal Cm that shows a short-time high level when the value of the slope forming voltage error integration signal Sva is equal to or greater than the value of the second variable calculated value signal Cb. Immediately after this, the slope forming voltage error integration signal Sva is reset to zero. Accordingly, the comparative signal Cm is a signal that shows a short-time high level for each pulse period Tpb.
The timer circuit MM outputs a timer signal Mm that shows a high level only during a period determined by the peak period setting signal Tps from the time point at which the comparative signal Cm shifts to a high level. Accordingly, this timer signal Mm is a signal that shows a high level during a predetermined peak period, and that shows a low level during the subsequent base period. The switching circuit SW is switched by this timer signal Mm, and outputs the peak current setting signal Ips or base current setting signal Ibs as the current waveform setting signal Ifs. The current detection circuit ID detects the welding current io, and outputs a current detection signal id. The current error amplifying circuit EI amplifies the error between the current waveform setting signal Ifs and the current detection signal id, and outputs a current error amplifying signal Ei. Accordingly, a welding current io that corresponds to the current waveform setting signal Ifs is caused to flow.
FIG. 13 is a timing chart of the respective signals of the welding power source. FIG. 13(A) shows the variation over time of the welding current io, FIG. 13(B) shows the variation over time of the welding voltage vo, FIG. 13(C) shows the variation over time of the slope forming voltage error integration signal Sva, FIG. 13(D) shows the variation over time of the comparative signal Cm, and FIG. 13(E) shows the variation over time of the timer signal Mm. The following description will make reference to the figure.
When the n-th pulse period Tpb(n) begins at time t(n), the comparative signal Cm varies to a high level for a short time as shown in FIG. 13(D). Accordingly, as shown in FIG. 13(E), the timer signal Mm shifts to a high level for the duration of a predetermined peak period Tp. When this timer signal Mm is at a high level, a peak current Ip flows as shown in FIG. 13(A), and a peak voltage Vp is applied as shown in FIG. 13(B). Furthermore, the integration of Sva is initiated at time t(n) as shown in FIG. 13(C).
As shown in FIG. 13(E), the timer signal Mm shifts to a low level when the predetermined peak period Tp has elapsed. Consequently, as shown in FIG. 13(A), a base current Ib flows; furthermore, a base voltage Vb is applied as shown in FIG. 13(B). When SVA≧B at time t(n+1), the comparative signal Cm again shifts to a high level for a short time as shown in FIG. 13(D), and the (n+1)-th pulse period Tpb(n+1) is begun. Welding power source output control is performed by repeating the operation, so that the external characteristics of the Equation (1) are provided. Prior art documents relating to the above technique include Japanese Patent Application Laid-Open No. 2002-361417, for example.
As described above, in pulse arc welding output control methods of the prior art, external characteristics with a desired slope can be provided. However, as a prerequisite condition for that, the peak current Ip in the peak period and the base current Ib in the base period need be constant. In the case of the direct-current pulse arc welding described above with reference to FIG. 10, this prerequisite condition is satisfied so that the conventional method is applicable. However, in the case of the alternating-current pulse arc welding that will be described below, the base current Ib during the base period does not remain constant, but varies, and therefore the prerequisite condition is not met. Consequently, the conventional method cannot be used for the alternating-current pulse arc welding.
FIG. 14 is a current-voltage waveform diagram of the alternating-current pulse arc welding. FIG. 14(A) is a waveform diagram of the welding current io, FIG. 14(B) is a waveform diagram of the absolute value ia of the welding current, FIG. 14(C) is a waveform diagram of the welding voltage, and FIG. 14(D) is a waveform diagram of the absolute value va of the welding voltage. The following description will make reference to these diagrams.
In the alternating-current pulse arc welding, the voltage polarity in a partial period (time t21 to time t22) in the base period Tb from time t2 to time t3 is reversed, so that a minus electrode period Ten is formed. Specifically, during the first base period Tb1 from time t2 to t21, a first base current Ib1 with an electrode plus polarity flows as shown in FIG. 14(A), and a first base voltage Vb1 is applied as shown in FIG. 14(C). Then, during the minus electrode period Ten from time t21 to t22, a minus electrode current Ien with a minus electrode polarity flows, and an electrode minus voltage Ven is applied. Ten, during the second base period Tb2 from t22 to t3, a second base current Ib2 with an electrode plus polarity again flows, and a second base voltage Vb2 is applied.
The welding current mean value Iw and welding voltage mean value Vw in the case of such an alternating-current waveform are defined as follows. As shown in FIG. 14(B), the mean value of the absolute value ia of the welding current for each pulse period is the welding current mean value Iw. Similarly, as shown in FIG. 14(D), the mean value of the absolute value va of the welding voltage or each pulse period is the welding voltage mean value Vw. Accordingly, it is necessary to perform output control so that the intersection point of the welding current mean value Iw and welding voltage mean value Vw during the pulse period Tpb from time t1 to t3 is located on the desired external characteristic.
In the alternating-current pulse arc welding, as shown in FIG. 14(B), the base current Ib1 during the base period Tb varies as Ib1, |Ien|, Ib2, and is thus not a constant value. Accordingly, the prerequisite condition of the conventional method is not met, and therefore the conventional method is not applicable to the alternating-current pulse arc welding.